Mechanical presses are commonly used to produce stamped car parts from steel blanks or workpieces. Traditionally the press drive and power transmission system, or kinematics is driven via a flywheel. The function of the flywheel is to store the necessary energy to make a cycle. The flywheel is connected and disconnected to the kinematics by means of a clutch and brake system (which may be pneumatic or hydraulic). When a die or mould of such presses is to be changed, the setup involving setting the mechanical positions to compensate for wear over time etc. is usually a complicated and lengthy process including mechanical adjustments that require a great deal of skill and knowledge. Maintenance is also required for any clutch or brake in the drive chain.
In addition, once setup to run with a given die, the working cycles of traditional motor driven mechanical presses, link presses, crank presses and similar are fixed. For example once the speed of the flywheel is set and the clutch engaged, the press will move following a fixed pattern, such as that of FIGS. 3, 5 (Prior Art) repeated as many times as required. Press speed is described here in terms of a rotational speed of the motor or of a mechanical transmission part such as an eccentric, or linear speed of the press ram. Having a fixed cycle means that any adjustment to, or optimization of, the press working cycle requires an interruption of production and adjustment of the mechanical components of the drive transmission, the flywheel etc. in order to modify the press cycle or cycle time. FIG. 3 (Prior Art) shows a general diagram for a press production cycle of a traditional mechanical press, a speed profile, expressed in terms of an eccentric speed Wp against time. Production cycle time, the total time for a complete production cycle from start to finish and including a press cycle, normally includes a short acceleration time at the beginning of the press cycle up to pressing speed Wp, a time period at constant pressing speed Wp, a time period P during the actual pressing operation when speed normally drops, a time after pressing during which speed gradually increases back to pressing speed, and finally a time period of deceleration or braking when the press is brought to a standstill at the end of the press cycle. Finally and usually when the press is being unloaded and re-loaded, the press normally remains at a standstill for a period of time. The production cycle thus begins with the start of a press cycle and ends at the end of a press cycle plus any standstill time.
The press is normally brought to a standstill by mechanical braking. FIG. 5 (Prior Art) shows a general position profile diagram including a complete production cycle expressed in terms of ram or slide position against time. The position profile shows a cycle which begins with ram position at Top Dead Centre (TDC) and the ram then accelerating to a lower position until a pressing stage P beginning with point of impact I between press die and workpiece. The ram continues to move downward to Bottom Dead Centre (BDC), the lowest ram position, press fully closed. After BDC the ram is accelerated back up toward TDC at which point it is in a fully open position again.
U.S. Pat. No. 6,619,088 to Oyamada entitled Bottom Dead Centre Correction Device for Press Servo Machine describes a bottom dead centre correction device for a servo press machine in which a slide is raised and lowered with a servo motor as the power source. Such a press has the advantage of not requiring a flywheel to provide energy for a pressing or stamping operation. A press with this type of transmission generally has a high peak power requirement. However, the device shown has a ball-screw drive which type of transmission up till now has usually been limited in the amount of force that may be transmitted and limited by cost of manufacture to small presses and/or for stamped products of high value.
It is known from the publicity material of Aida-America Corporation to drive a mechanical press using a servo motor with a direct drive to the slide mechanism (Ref 1.) This type of servo press with a direct drive has the advantage of requiring no flywheel, clutch or brake and having a programmable slide motion. However, servo motor presses may have a high peak power consumption for some products, especially large products requiring deep drawing.